The dealer probably just reset the light because this code normally comes when you don't tighten the gas cap properly. My SUV is a Suzuki and it threw the evap code when new. The dealer wanted to demonstrate how to tighten the gas cap to us. My late wife got tired of it and told me to take it in myself. I talked them into running a smoke test. They discovered the pump that was to pull vacumn to check the system was bad. It was replaced and no more codes.
I would get the dealer to state in writing that they will fix this if it happens again or ask them for a smoke test now. All they have to do is put the car on the lift and get their smoke pen.